The majority of decks constructed in North America are constructed from wood, this includes the support structure as well as the decking surface. However, with age and exposure to moisture, wood can split, warp, twist, splinter and rot. These effects are most apparent on the horizontal decking surfaces where water can collect, especially if the deck boards become cupped. Recently, a number of manufacturers have started offering extruded profiles made from moisture resistant materials which can be used as an alternative to wood decking in the construction of decks. These products, such as those produced by Trex Company Inc., Winchester, Va., and Advanced Environmental Recycling Technologies Inc. (AERT), Springdale, Ariz., are said to offer a number of advantages over wood, particularly relating to the moisture resistance of the materials used in their manufacture.
The majority of these alternative decking products are currently being produced from composite materials which consist of thermoplastic resins that contain a high percentage by weight (30-70%) of cellulosic fillers. These fillers are typically derived by the comminution or attrition by grinding or milling of wood, plant matter or agricultural byproducts such as hulls, husks, shells and straws to produce discrete cellulosic fibers or cellulosic particles. Cellulosic fibers which are a byproduct of paper production or recycling are also being used as fillers. In addition to being cheaper than glass fibers (a common reinforcing filler for thermoplastic resins), cellulosic fibers are typically much lower in cost than the thermoplastic resins in which they are used as fillers, so a higher cellulosic fiber content in the resin used to produce the alternative decking product results in a lower cost product. Higher cellulosic fiber content can also result in improved mechanical properties such as strength and stiffness. These composite materials generally show very good moisture resistance and aging characteristics in comparison to wood. However, too high a cellulosic fiber content can result in products which may not be as moisture resistant as desired. In this application, it is these thermoplastic resin cellulosic fiber composite materials which are being referred to when the term composite is used.
While there are a growing number of manufacturers of these composite decking products, most of these manufacturers recommend against using their products as structural members such as joists or posts. Typically, the manufacturers of the composite decking products recommend using wood to construct the structure on which the composite decking product is installed. This results in a decking surface which may have a lifetime guarantee, while the wooden structure supporting it is still prone to moisture damage and may need replacement if the damage is severe enough. The effects of moisture on the wooden support structure can be minimized by using naturally moisture resistant wood species such as cedar or redwood, which are usually sold at a substantial premium to less moisture resistant species. A more economical solution has been to use treated lumber as the structural members with the composite decking products. However, the effect of the treatment will decrease over time as the treatment chemicals leach out of the wood. As such, using moisture resistant wood species and pressure treated lumber will delay the decay of the wood, but it will not prevent splitting, warping, twisting and splintering of the wood which is caused by repeated cycles of the wood getting wet and drying out and can significantly weaken the structural members.
In February of 2002, the United States Environmental Protection Agency announced a phaseout of chromated copper arsenate (CCA) treated lumber by the treated lumber industry. At the time, CCA treated lumber accounted for over 80% of pressure treated lumber sold in North America. The phase out was the result over the concerns over the toxicity of the CCA and the fact that it can readily leach out from lumber and contaminate nearby soil. Other chemical preservatives are currently available, with the most likely successor to CCA being alkaline copper quaternary (ACQ), which is substantially more expensive than CCA, and will result in higher treated lumber prices. The various chemical formulations used in treating lumber typically act as fungicides which enhance the moisture resistance of the wood by killing fungi which can lead to rot and decay. However, according to the Canadian Environment Ministry, all chemical wood preservatives are classified as pesticides as they achieve decay control as a result of their significant toxicity, and that while the potency of the various preservatives varies, all are poisonous to some degree and are potentially hazardous to humans and other forms of life. In addition, the announced phase out of CCA treated lumber has resulted in increased prices for lumber from moisture resistant wood species such as cedar, as a result of increasing demand.
While a number of composite decking product manufacturers are promoting composite railing posts (which is considered a structural application), currently there is only one type of product which is being promoted for use as structural framing members to replace wood framing in building decks, and that is glass fiber reinforced high density polyethylene (HDPE) plastic lumber, such as that produced by US Plastic Lumber Ltd., Boca Raton, Fla. These products are solid and mimic the sizes and shape of standard lumber profiles (i.e. 2×6, 2×8, etc.). However, as a result of the significantly higher density of these products, they are substantially heavier than wood of the same size. In addition, these products can be difficult to cut and drill and can quickly dull saw blades and drill bits as a result of the glass fiber content. Finally, because of the relatively high cost of the glass fiber reinforcement, the cost of these products can be many times that of wood even when they are produced using recycled HDPE.
In producing composite structural members, their weight might also be a concern, as the density of the composite materials is typically significantly higher than that of wood. As a result, composite structural members may be substantially heavier than similarly sized wood structural members. One way to reduce the weight and consequently the cost of the composite structural member is to reduce the amount of material used in their production by making hollow profiles rather than a solid ones. This offers an additional advantage, since reducing the weight of the structural member reduces the load on any support structure for the structural member. However, despite the fact that the composite materials can be substantially stiffer than the thermoplastic resins that they are made from, the stiffness (as measured by the flexural modulus) of the composite materials may be only 10% to 50% of that of wood. As a result, removing material to produce a hollow composite profile may result in a product which may not have the strength and stiffness required for its use as a structural member and thus the composite member would have to be substantially larger than the wood structural member it is meant to replace. Further, such a composite member may result in allowable spans (as for a joist) which are unacceptably short.
One way to overcome this situation, is to reinforce the hollow composite profiles. This can be done by using materials, which have much better mechanical properties (i.e. stronger and stiffer) than the composite materials, in conjunction with them. One option is to insert reinforcing elements such as metallic rods or tubing or pieces of wood into the hollow cavities of the composite profile, as is recommended by some manufacturers of composite railing posts. The reinforcing capacity of these types of inserts is substantially reduced by the fact that typically there is no actual bond between the reinforcing element and the composite profile, beyond a friction fit. Therefore, reinforcing materials which are bonded in some way to the composite profiles would result in potentially stronger and stiffer structural members. Particularly, if these reinforcing elements are located where the greatest stresses are experienced in the specific structural member. In general, the outside surfaces of the structural member furthest from the center of mass experience the greatest stresses, for example the top and bottom surface of a joist. Coincidentally, reinforcing materials located furthest from the center of mass of the object to be reinforced yield the greatest reinforcing effect for a given amount material. Therefore, placing reinforcing materials on the outside surfaces of the structural member furthest from the center of mass, such as the top and bottom surfaces of a joist, would be the most effective and efficient approach.
Therefore it would be desirable to have a product which could be used to replace untreated lumber, pressure treated lumber, cedar and redwood used as structural members in the construction of decks and other outdoor constructions, particularly in conjunction with moisture resistant composite decking products. Preferably, the product would have the same moisture resistant characteristics of the decking products. It would be easy to work with (i.e. have the workability of wood) and easy to install. The product would also be as similar as possible to the wooden structural members it is meant to replace in terms of weight, mechanical properties and cost, if possible. The product would make use of reinforcing materials which are bonded to the outside surfaces furthest from the center of mass of the product to achieve greater strength and stiffness. In addition the product would have hollow sections in order to reduce the weight of the product and the amount of material used in its manufacture, thereby reducing its cost. It is also desirable to have processes which could be used to produce these reinforced composite structural members.